Dear Hugs users,
could you answer the following question from a Haskell novice?
In "Yet another Haskell Tutorial" by Hal Daumé III there is the following
exercise:
Exercise 4.8 Write functions listHead, listTail, listFoldl and listFoldr
which are equivalent to their Prelude twins, but function on our List datatype.
Don’t
worry about exceptional conditions on the first two.
My question is solely on listfoldr, in fact I only want to ask whether the
solution provided in the tutorial is really so obviously wrong as I perceive it
to be:
(What follows uses the definition
data List a = Nil
| Cons a (List a)
)
listFoldr f y Nil = y
listFoldr f y (Cons x xs) = f x (foldr f z xs)
This IS wrong, isn't it? The y does not even appear after all.
And what can you say about my attempt
for listFoldr?
listFoldr f y Nil = y
listFoldr f y (Cons x xs) = f y (listFoldr f (f y x) xs)
My attempt yields a similar, albeit not identical result compared with the
Prelude function foldr when :t-ing it under Hugs:
Hugs> :t foldr
foldr :: (a -> b -> b) -> b -> [a] -> b
Hugs> :t listFoldr
listFoldr :: (a -> a -> a) -> a -> List a -> a
Thank you very much.
Christian
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